High quality insulation systems are often used to prevent heat leak into production, distribution or storage facilities for material and equipment intended to be kept at low temperatures. This is particularly the case where the low temperatures required are cryogenic temperatures, i.e., below 240.degree. K. Such insulation systems frequently employ double-walled means with the volume or space between the walls evacuated to relatively low vacuum. This is advantageous because by reducing the number of gas molecules in the space, the frequency of intermolecular collisions, or equivalently, the amount of heat transferred by gas conduction, is reduced. Often the evacuated space also contains solid insulation such as glass fibers, perlite or superinsulation to further retard heat transfer.
The gases to be evacuated from the space are from several sources including gas present during assembly of the double-walled insulating means, offgassing from materials exposed to the vacuum and leakage or permeation into the evacuated space after the space is sealed. Typically the desired vacuum is produced by a combination of external vacuum pumping and an adsorbent within the evacuated space. External vacuum pumping removes much of the gas present during assembly while adsorbent removes gases from offgassing, permeation and leakage thus achieving and maintaining a low vacuum for effective heat transfer resistance.
Adsorbents are normally effective in removing from the evacuated space gases such as hydrocarbons and atmosphere gases such as oxygen, nitrogen, and water vapor. However, a problem with adsorbents is that their affinity for water plays havoc with their effectiveness in adsorbing other gases. Water is very strongly held by the commonly used adsorbents such as zeolite molecular sieve or activated carbons. For example, zeolite 5A, a conventional molecular sieve adsorbent, has a water capacity of about 21 percent by weight, at which point it has essentially no capacity for oxygen and nitrogen. Activated carbon is not as hydrophilic as molecular sieve at low humidity, but it will become saturated on exposure to higher humidity. Carbon holds water rightly and loses half its capacity for air at about 20 percent water loading. Essentially all air capacity is lost at about 40 percent water loading. Also, carbon has the major disadvantage of combustibility; it is not safe for use in insulation systems having potential for high concentration of oxygen, e.g., a liquid oxygen storage tank.
Several approaches have been taken to solve this water problem. The most straightforward approach is to simply use enough adsorbent so that it is not all saturated by water. In most cases, the amount required is so large as to be impractical.
A second method is to prevent water from entering the system. For example, the adsorbent can be stored in a sealed container prior to use. However keeping the insulation dry is generally quite complicated. There is generally a certain amount of water structurally associated with the insulation, and, due to its large surface area, insulation adsorbs a significant amount of water from the ambient air. The exact amount of adsorbed water depends on the relative humidity of the air, but powder or glass fiber insulation typically may contain from 0.5 to 1.0 percent weight of water, and other fibers, such as rayon, may contain up to 6 percent of adsorbed and/or dissolved water. Protecting the typically large amount of insulation from exposure to humidity is cumbersome and adds to the cost of manufacturing the system.
A third means for reducing water content in an insulation system is simultaneous heating and evacuation of the system. This costly method cannot completely remove water from the insulation, since all the water does not diffuse to the point of evacuation at a fast enough rate, e.g., within several days. Furthermore heating may destroy some of the materials used in constructing the system. Also large containers, such as the outer vessel of a liquid storage tank, are very difficult and often impractical to heat.
Usually, a combination of these three methods is used as shown for example in U.S. Pat. No. 4,154,363--Barthel. Barthel uses activated carbon in combination with multilayer insulation composed of hydrophobic fibers and metallic foil. The insulation space undergoes a short evacuation period. Since the fibers are hydrophobic, they evolve only a small amount of water, all of which can be adsorbed by the quantity of adsorbent present. Barthel demonstrates one way to prevent saturation of the adsorbent with water, but the multilayer insulation used is expensive and complex to manufacture. It would be desirable to use a conventional insulation that is simple and low in cost.
It is therefore an object of this invention to provide an improved insulation system.
It is another object of this invention to provide an improved insulation system for use at vacuum conditions wherein water does not detrimentally affect the ability of adsorbent to attain and maintain vacuum conditions.
It is a further object of this invention to provide a method to fabricate double-walled insulating means without need for a water removal step.